nullable syntax update

[andrewrk]

• Replace a ?? b binary operator with a ifnull b. After this change, all control flow occurs with keywords only.
  • other suggestions instead of ifnull are welcome. ifnull is nice because it is 2 lowercase words squished together so nobody will miss the identifier name.
• Remove ??x prefix operator. Replace it with x.?. After this change and Pointer Reform #770, this symmetry exists:
  • *T is used to make a pointer type, and x.* derefs
  • ?T is used to make a nullable type, and x.? de-nullifies
  • Both operations have assertions that are potentially unsafe - .* asserts the pointer is valid among other things and .? asserts the value is non-null.

Anecdotally, this will look a lot better syntactically, especially with C-translated code.

[phase]

Will there be anything for null function call chaining like Kotlin’s ?.

ifnull is rather verbose, Kotlin uses ?: which is short and simple.

[thejoshwolfe]

Will there be anything for null function call chaining like Kotlin’s ?.

Feel free to propose something in a separate issue. I don't remember if this one has been proposed so far.

[BraedonWooding]

I really like the x.? change but I'm not so sure about a ifnull b, while I understand the idea of removing keywords from control flow I'm worried that one it reads a little odd, since it does read like do a if b is null, and two it adds a bit of confusion as it is a two word keyword which is generally not done since it isn't as readable (like in this case it is quite verbose) and I'm worried that people will think the whole ifx pattern will be a consistent thing such as elsenull, ifnotnull (etc).

Perhaps something simpler like a.? else b, or like; a.? : b in similarity to the common ternary operator a != null ? a : b. But the second option I guess introduces another symbol which is kind of what is trying to be eliminated here.

I'm not particularly against it but I'm not all for it.

[PavelVozenilek]

?? is very easy to type and it stands out. if? or if?? could be used if keyword is that needed.

What about reusing or? It's short, and a ?? b is pretty similar to a or b:

• a or b when a and b are bools means "evaluate a and return it if it's true, otherwise evaluate and return b".
• a ?? b when a is ?T and b is T or ?T means "evaluate a and return it if it's non-null, otherwise evaluate and return b".

[phase]

I like @hcff’s idea but nullable booleans might get confusing.

[tgschultz]

Should x.! then unwrap an error union, equivalent to x catch unreachable?

Also not a huge fan of ifnull. To throw some options out there and maybe inspire some better ones:

a fail b;
a handle b;
a provide b;
a fill b;
a check b;
a coalesce b;
a alt b;
a otherwise b;

[eira-fransham]

My personal favourite would be a.? else b. It doesn't introduce a new keyword and it's (IMO) very readable. If a.! was introduced then the question is whether a catch b should be a.! else b (removing a keyword is always good when possible in my opinion).

[raulgrell]

Would there be an issue with using a straight a else b?

[thejoshwolfe]

Would there be an issue with using a straight a else b?

yes. it would cause ambiguity in if (a) b else c else d. we can't use a else b for null coalescing.

[raulgrell]

@thejoshwolfe - Damn, I should have seen that.

One of the proposals in #760 mentioned the possibility of removing the parentheses around the condition of if and while. I'm not sure if this implied the use of curly brackets around the body of the conditional/loop would be required - if that's the case, would that be enough to resolve the ambiguity or does another one pop up?

Your example would have to be expressed as either

if a { b } else { c else d }

or

if a { b else c } else { d }

[akavel]

Another bikeshed proposals (sorry if something conflicts with existing keywords/...):

a ornull b
a fallback b
a default b
a withdefault b